bug#21782: 25.0.50; New functions nfront/front

[Constantino Calancha]

> ISTR there are traditional names for those functions.
> I don't remember what they were, but if we are going to add them,
> we should use their traditional names.

Sure, i also prefer keep their original names.

Considering my previous example: i disagree with myself about
nfront being consistent with nbutlast.

nfront should return nil when N<= 0 as seq-take does.
Only when N is nil is when we provide the sensible default N=1 as nbutlast.